1. Field of the Invention
This invention relates to an optical tomography imaging method and apparatus, wherein a signal light beam, which is a low coherence light beam, is irradiated to a measuring site, and a tomographic image of the measuring site is thereby acquired. This invention particularly relates to an optical tomography imaging method and apparatus, wherein fine structure information at a surface of a measuring site and a deep portion of the measuring site is imaged in accordance with a reflected light beam of a signal light beam.
2. Description of the Related Art
Optical tomography imaging apparatuses utilizing a low coherence light beam, particularly optical tomography imaging apparatuses, in which an optical intensity of a low coherence interference light beam is measured with a heterodyne detection technique and a tomographic image of a measuring site is thereby obtained, have heretofore been utilized for acquiring optical tomography images of fine structures under the eyeground retina.
With the optical tomography imaging apparatuses, a tomographic information is acquired by dividing a low coherence light beam, which has been radiated out from a light source comprising a super luminescent diode (SLD), or the like, into a signal light beam and a reference light beam, slightly shifting a frequency of the reference light beam with a piezo-electric device, or the like, irradiating the signal light beam to a measuring site, causing reflected light beam of the signal light beam, which has been reflected from a predetermined depth in the measuring site, and the reference light beam to interfere with each other, and measuring the optical intensity of the interference light beam with the heterodyne detection technique. The information at the deep portion of the measuring site has heretofore been acquired by slightly moving a moving mirror, or the like, which is located in an optical path of the reference light beam, thereby altering an optical path length of the reference light beam, and causing the optical path length of the reference light beam and the optical path length of the signal light beam to coincide with each other.
In the optical tomography imaging apparatuses described above, in order for the tomographic information at the desired depth in the measuring site to be obtained, it is ideal that the interference of the signal light beam and the reference light beam with each other occurs only when the optical path length of the reference light beam and the optical path length of the signal light beam perfectly coincide with each other. However, actually, in cases where the difference between the optical path length of the signal light beam and the optical path length of the reference light beam is equal to at most a coherence length determined by the light source, the interference of the signal light beam and the reference light beam with each other occurs. Specifically, resolution in the low coherence interference is determined by the coherence length, which is determined by the light source.
In cases where a wavelength distribution of the light beam produced by the light source is a Gaussian distribution, a coherence length xcex94L is represented by Formula (1) shown below.
xcex94L=(2/xcfx80)xc2x7ln2xc2x7(xcexc/xcex94xcex)xe2x80x83xe2x80x83(1)
wherein xcexc represents the center wavelength, and xcex94xcex represents the spectral width.
For example, in cases where a SLD producing a light beam having a center wavelength of 800 nm and a spectral width of 20 nm is employed as the light source, the coherence length becomes equal to approximately 14 xcexcm. Therefore, in cases where the SLD having the characteristics described above is employed as the light source in the conventional optical tomography imaging apparatuses described above, the resolution becomes equal to approximately 14 xcexcm. Accordingly, with the conventional optical tomography imaging apparatuses described above, in cases where the measuring site has a plurality of layers falling within a thickness approximately equal to the resolution, reflected light beams coming from the respective layers could not be discriminated from one another.
Recently, in the clinical fields, usefulness of tomographic images of living body tissues, and the like, has been known widely, and it is desired that, besides tomographic images of the eyeball site, tomographic images of living body tissues, which exhibit higher light scattering than the eyeball site, be acquired with a high resolution. For such purposes, it is necessary to utilize a light source, which is capable of producing a low coherence light beam having a high intensity and a short coherence length. However, with the SLD, the out put cannot always be enhanced. Also, with the SLD, the problems occur in that, since the spectral width is determined by a band gap, the spectral width cannot be set at a large value and the coherence length cannot be set to be short.
Therefore, an optical tomography imaging apparatus, in which a light source provided with a KLM mode-locked Ti:sapphire laser is utilized, has been proposed in, for example, xe2x80x9cOptics Letters,xe2x80x9d Vol. 21, No. 22, pp. 1839 to 1841, by B. E. Bouma, et al., 1996. With the proposed optical tomography imaging apparatus, a low coherence light beam having a high intensity and a wide spectral width is obtained by the utilization of an ultrashort pulsed light beam and dispersion delay of an optical fiber and is utilized as a signal light beam and a reference light beam, such that a tomographic image is capable of being acquired with a high resolution.
However, the optical tomography imaging apparatus described above, in which the light source provided with the KLM mode-locked Ti:sapphire laser is utilized, has the problems in that the light source section is large in size, high in cost, and hard to process. Thus the optical tomography imaging apparatus described above, in which the light source provided with the KLM mode-locked Ti:sapphire laser is utilized, practically has the problems with regard to the size, the cost, and the processing of the apparatus.
The primary object of the present invention is to provide an optical tomography imaging method for acquiring a tomographic image by the utilization of low coherence interference, wherein a light source, which is large in size, high in cost, and hard to process, need not be utilized, and tomographic information is capable of being acquired with a high resolution.
Another object of the present invention is to provide an apparatus for carrying out the optical tomography imaging method.
The present invention provides an optical tomography imaging method, comprising the steps of:
i) dividing a low coherence light beam into a signal light beam and a reference light beam,
ii) irradiating the signal light beam to a measuring site,
iii) shifting a frequency of the reference light beam to a frequency having a slight frequency difference from the frequency of the signal light beam,
iv) causing a reflected light beam of the signal light beam, which has been reflected from a predetermined deep portion of the measuring site, and the reference light beam to interfere with each other, an interference light beam being thereby obtained,
v) measuring an intensity of the interference light beam, and
vi) acquiring an optical tomography image of the measuring site in accordance with the measured intensity of the interference light beam,
wherein the low coherence light beam is an amplified spontaneous emission light beam, which is radiated out from an optical fiber having been doped with a light emitting material when excitation energy is applied to the optical fiber.
The present invention also provides an optical tomography imaging apparatus, comprising:
i) means for dividing a low coherence light beam into a signal light beam and a reference light beam,
ii) means for irradiating the signal light beam to a measuring site,
iii) means for shifting a frequency of the reference light beam to a frequency having a slight frequency difference from the frequency of the signal light beam,
iv) means for causing a reflected light beam of the signal light beam, which has been reflected from a predetermined deep portion of the measuring site, and the reference light beam to interfere with each other, an interference light beam being thereby obtained,
v) means for measuring an intensity of the interference light beam, and
vi) means for acquiring an optical tomography image of the measuring site in accordance with the measured intensity of the interference light beam,
wherein the low coherence light beam is an amplified spontaneous emission light beam, which is radiated out from an optical fiber having been doped with a light emitting material when excitation energy is applied to the optical fiber.
The term xe2x80x9clight emitting materialxe2x80x9d as used herein means a material having properties such that the material is capable of being excited by excitation energy applied from the exterior, and the excitation energy is thereby radiated out as light from the material. Also, the term xe2x80x9creflected light beam of a signal light beam having been reflected from a predetermined deep portion of a measuring sitexe2x80x9d as used herein means both the reflected light beam, which has been reflected from the predetermined deep portion of the measuring site, and the reflected light beam, which has been reflected from the surface of the measuring site.
The term xe2x80x9cmeasuring an intensity of an interference light beamxe2x80x9d as used herein means the measurement of the intensity of a beat signal (i.e., the interference light beam), the intensity of which repeatedly becomes high and low at a frequency equal to the difference between the frequencies of the signal light beam and the reference light beam. By way of example, the measurement may be made with a heterodyne interferometer.
In the optical tomography imaging method and apparatus in accordance with the present invention, the measuring site should preferably be a site of living body tissues, and
a wavelength of the low coherence light beam should preferably fall within the range of 600 nm to 1700 nm.
Also, the excitation energy should preferably be energy of excitation light having a wavelength falling with in a wave length region of 500 nm to 1700 nm.
Further, the light emitting material should preferably be a dye capable of producing fluorescence.
Furthermore, the light emitting material may be selected from the group consisting of transition metal ions, rare earth element ions, and complex ions.
Also, the light emitting material may be at least one kind of ion selected from the group consisting of Cr3+, Mn4+,Mn2+, Fe3+, which are transition metal ions; Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+, which are rare earth element ions; and WO42xe2x88x92, MoO42xe2x88x92, VO43+, Pt(CN)42xe2x88x92, and WO66xe2x88x92, which are complex ions.
With the optical tomography imaging method and apparatus in accordance with the present invention, as the low coherence light beam for forming the signal light beam and the reference light beam, the amplified spontaneous emission light beam, which is radiated out from the optical fiber having been doped with the light emitting material when the excitation energy is applied to the optical fiber, is utilized. Therefore, the low coherence light beam having a high intensity and a wide spectral width is capable of being obtained from a light source, which is small in size, low in cost, and easy to process. Accordingly, the light source provided with an ultrashort pulse laser, or the like, which light source was necessary in the conventional optical tomography imaging apparatuses and is large in size, high in cost, and hard to process, need not be utilized, and the resolution in low coherence interference is capable of being enhanced.
Also, with the optical tomography imaging method and apparatus in accordance with the present invention, wherein the measuring site is a site of living body tissues, and the wavelength of the low coherence light beam falls within the range of 600 nm to 1700 nm, the signal light beam has desirable transmission characteristics and desirable scattering characteristics at the measuring site. Therefore, a desired tomographic image is capable of being acquired.
Further, with the optical tomography imaging method and apparatus in accordance with the present invention, wherein the excitation energy is energy of the excitation light having a wavelength falling within the wavelength region of 500 nm to 1700 nm, the light emitting material, which has been doped in the optical fiber, is capable of being excited efficiently.
Furthermore, with the optical tomography imaging method and apparatus in accordance with the present invention, wherein the light emitting material is the dye capable of producing the fluorescence, the low coherence light beam having a desirable center wavelength and a desired spectral width is capable of being obtained.
Also, with the optical tomography imaging method and apparatus in accordance with the present invention, wherein the light emitting material is selected from the group consisting of transition metal ions, rare earth element ions, and complex ions, the light emitting material is capable of being easily doped in the optical fiber. Further, the low coherence light beam having a desirable center wavelength and a desired spectral width is capable of being obtained.
Furthermore, with the optical tomography imaging method and apparatus in accordance with the present invention, the light emitting material may be at least one kind of ion selected from the group consisting of Cr3+, Mn4+, Mn2+, Fe3+, which are transition metal ions; Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+, which are rare earth element ions; and WO42xe2x88x92, MoO42xe2x88x92, VO43+, Pt(CN)42xe2x88x92, and WO66xe2x88x92, which are complex ions. In such cases, the excitation light is capable of being efficiently converted into the desired low coherence light beam.